Ink jet printing is utilized in a variety of printing applications and is particularly useful in the assembly of magazines where customized or personalized messages, such as address labels, need to be printed without smearing. Ink jet printing devices of the continuous stream type commonly employ a print head having a droplet generator with multiple nozzles from which continuous streams of ink are emitted under pressure. The droplets of ink are charged and deflected by an electromagnetic field into a gutter for ink collection or alternatively to a specific target location such as onto paper which may be continuously transported at a relatively high speed across the paths of the droplets. Ink is delivered to the ink jet printing heads from an ink supply tank, The ink which is deflected by the electromagnetic field into the gutter is drawn by a vacuum pump through a vacuum line back into the ink supply tank. The ink returning to the ink supply tank is entrained in air which is vented from the ink supply tank.
The ink used in an ink jet printer commonly includes a highly volatile solvent such as methyl ethyl ketone (MEK). The air vented from a conventional ink supply tank includes significant amounts of solvent vapors, and such prior art ink jet printers discharge substantial amounts of solvent into the atmosphere. Such solvents are expensive materials in the printing process, and evaporation losses comprise a significant amount of the solvent used in an ink jet printing process. Discharge of solvents into the atmosphere is also harmful to air quality.
In some prior art ink jet printers, the air vented from the ink supply tank is passed through a room temperature container wherein solvent vapors can condense and can be returned to the ink supply tank. Such systems are effective to return only a small quantity of solvent to the ink supply tank, and a substantial quantity of solvent is vented as vapor to the atmosphere.
Since solvent and ink are in the printing process, the ink supply tank must be refilled. Although the level of ink in the ink supply tank can be monitored by periodic visual inspections, the ink supply tank typically includes a level indicator for monitoring the level of ink. Additionally, the ink must be maintained within a specific viscosity range in order to provide satisfactory printing results. Therefore, the viscosity of ink supplied from the ink supply tank to the print head typically is monitored with a viscosity meter. To prevent the tank from running empty and to maintain the ink viscosity within a desired range, separate ink and solvent make up containers are typically utilized. The ink and solvent make up containers are connected to the ink supply tank by respective conduits, and each conduit includes a respective control valve. Each of the respective ink and solvent control valves is moveable between a closed position and an open position wherein ink or solvent, respectively, flows from the respective make up container and into the ink supply tank.
If the ink in the ink supply tank falls below a predetermined level, a control system connected to the level indicator causes the ink control valve to open so that ink flows from the ink make up container into the ink supply tank to replenish the ink supply. If the viscosity of the ink rises above a predetermined level, the control system connected to the viscosity meter causes the solvent control valve to open so that solvent flows from the solvent make up container into the ink supply tank to decrease the viscosity of the ink.
Commonly, the levels of ink and solvent in the make up containers are monitored to ensure that the make up containers do not run empty. Although they may be monitored by periodic visual inspection, the make up containers typically include level indicators connected to indicator lights. The level indicators typically include one or more sensors extending through the top and walls of the make up containers. The make up containers typically also include tops which are removable for ink or solvent to be manually poured into the respective container. When the top of the solvent make up container is opened for filling the container, significant amounts of solvent are vented from the container to the atmosphere. The level indicator sensors can also be damaged when the tops are repeatedly removed and replaced.